The overall goal of this project is to elucidate molecular mechanisms that contribute to cardiac dysfunction in heart failure. This project focuses on one aspect of the disease, defects in excitation-contraction (EC) coupling. It is not the intention of the applicant to suggest that defects of EC coupling are the only mechanism underlying cardiac dysfunction in heart failure. The rationale for focusing on the role of EC coupling in heart failure is that cardiac contractility is activated by EC coupling and there is ample evidence in the literature from numerous laboratories indicating that EC coupling, and more broadly Ca2+ homeostasis, are altered in heart failure. Finally, the specific aims of this proposal focus specifically on one aspect of EC coupling, sarcoplasmic reticulum (SR) Ca2+ release which is regulated by the ryanodine receptor/Ca2+ release channel (RyR2). During the past 4 years of this project we have shown that RyR2 comprise a macromolecular complex in which PKA, PP1, PP2A are bound to the channel via targeting proteins that bind to RyR2 through leucine/isoleucine zippers (LIZs) enabling us to identify components of the RyR2 signaling complex and develop novel strategies for studying the function of the channel. The components of the RyR2 macromolecular complex, including PKA, PP1, PP2A, FKBP12.6 and PDE4D3 (phosphodiesterase), are potential novel therapeutic targets for heart failure. We have identified a novel defect in skeletal muscle EC coupling that may contribute to the impaired exercise tolerance in heart failure patients. We now propose the following aims: Aim 1) Determine the in vivo effects of defective regulation of RyR2 by PKA phosphorylation. Ser2809 is the site of PKA phosphorylation in RyR2. We showed that phosphorylation of 3/4 or 4/4 Ser2809 in the tetrameric RyR2 in failing heads causes depletion of FKVBP12.6 from the RyR2 macromolecular complex resulting in defective channels that exhibit increased open probability at low [Ca2+]cyt. Based on these findings we propose to test the hypothesis that PKA hyperphosphorylation of RyR2 and depletion of FKBP12.6 from the RyR2 macromolecular complex contribute to cardiac dysfunction in failing hearts. Aim 2) Determine the role of phosphodiesterases in defective RyR2 regulation in heart failure. Preliminary data summarized in this application show that the phosphodiesterase PDE4D3 is a novel component of the RyR2 macromolecular complex. Our preliminary data further show that PDE4D3 binds to RyR2 via the same targeting protein as PKA, mAKAP, and that the amount of PDE4D3 in the RyR2 complex is reduced in heart failure. On the basis of these preliminary findings we propose to test the hypothesis that defective regulation of PDE4D3 contributes to RyR2 PKA hyperphosphorylation in failing hearts. Aim 3) Identify mechanism(s) underlying impaired skeletal muscle function in heart failure. It is well established that heart failure patients have impaired exercise capacity that is not explained solely by the degree of cardiac dysfunction. Our preliminary data show that RyR1 in heart failure skeletal muscle are PKA hyperphosphorylated and depleted of the stabilizing protein FKBP12. In addition preliminary data show that heart failure skeletal muscle from a rat infarct model of failure exhibit early fatigue that correlates with the degree of RyR1 PKA phoshorylation. We propose to test the hypothesis that defective regulation of RyR1 due to PKA hyperphosphorylation and FKBP12 depletion contributes to impaired skeletal muscle function in heart failure.